Mapping the tandem mass spectrometric characteristics of citrulline containing peptides

Steckel, Arnold and Uray, Katalin and Turiák, Lilla and Gömöry, Ágnes and Drahos, László and Hudecz, Ferenc and Schlosser, Gitta (Vácziné) (2018) Mapping the tandem mass spectrometric characteristics of citrulline containing peptides. RAPID COMMUNICATIONS IN MASS SPECTROMETRY, 32 (11). pp. 844-850. ISSN 0951-4198

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Abstract

RATIONALE Protein citrullination (deimination) is a post-translational modification of proteins converting arginine(s) to citrulline(s). “Overcitrullination” could be associated with severe pathological conditions. Mass spectrometric analysis of modified proteins is hindered by several problems. A comprehensive study of fragmentation of deiminated peptides is not yet available. In this paper we have made an attempt to describe the characteristics of these processes, based on the studies of epitope model oligopeptides derived from clinically relevant proteins. METHODS Solution of purified model peptides containing either one or two citrulline residues as well as their native variants were injected directly to the electrospray source of a high accuracy and resolution quadrupole-time of flight instrument and were analysed by tandem mass spectrometry using low-energy collision induced dissociation. RESULTS Loss of isocyanic acid from citrulline residues is a preferred fragmentation route for deiminated peptides, which yields ornithine residues in the sequence. However, simultaneous detection of both the isocyanic acid loss and sequence fragments is often compromised. A preferential cleavage site was observed between citrulline and any other following amino acids yielding intensive complementary b and y type ions. Also, citrulline positioned at the C-termini displays a preferential cleavage N-terminal to this residue yielding characteristic y1 ions. These phenomena are described here for the first time and are referred to as the “citrulline effect”. CONCLUSIONS We found that the citrulline effect is very pronounced and could be used as a complementary tool for the confirmation of modification sites in addition to losses of isocyanic acids from the protonated molecules or from fragment ions. Low collision energy applied to peptide ions having partially mobile protons reveal the site of modification by generating specific and intensive fragments of the sequence. On the other hand, fragmenting parent ions with mobile protons usually allow full sequence coverage, although citrulline-specific fragments may exhibit lower intensities compared to other fragments.

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